diff --git a/CHANGELOG.md b/CHANGELOG.md
index f13e13c..79a7267 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,13 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [2.1.0] - 2026-09-04
+
+### Added
+
+- Add `invoices.paymentSummary(String id, double amount)` to get the related-document object needed to build a payment complement (complemento de pago): installment number, previous balance, and taxes prorated to the paid amount.
+- Add the `PaymentSummary` model.
+
 ## [2.0.0] - 2026-08-24
 
 ### Added
diff --git a/pom.xml b/pom.xml
index a69187f..de17cc0 100644
--- a/pom.xml
+++ b/pom.xml
@@ -5,7 +5,7 @@
 
   <groupId>io.facturapi</groupId>
   <artifactId>facturapi-java</artifactId>
-  <version>2.0.0</version>
+  <version>2.1.0</version>
   <name>facturapi-java</name>
   <description>Official Java SDK for Facturapi</description>
   <url>https://github.com/facturapi/facturapi-java</url>
diff --git a/src/main/java/io/facturapi/models/PaymentSummary.java b/src/main/java/io/facturapi/models/PaymentSummary.java
new file mode 100644
index 0000000..3244b70
--- /dev/null
+++ b/src/main/java/io/facturapi/models/PaymentSummary.java
@@ -0,0 +1,144 @@
+package io.facturapi.models;
+
+import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Related document summary for a payment complement (complemento de pago).
+ * Ready to be used as an element of the complement's related documents.
+ */
+@JsonIgnoreProperties(ignoreUnknown = true)
+public class PaymentSummary {
+  private String uuid;
+  private Integer folioNumber;
+  private String series;
+  private Integer installment;
+  private Double lastBalance;
+  private Double total;
+  private String currency;
+  private Double amount;
+  private List<PaymentSummaryTax> taxes = new ArrayList<>();
+
+  public String getUuid() {
+    return uuid;
+  }
+
+  public void setUuid(String uuid) {
+    this.uuid = uuid;
+  }
+
+  public Integer getFolioNumber() {
+    return folioNumber;
+  }
+
+  public void setFolioNumber(Integer folioNumber) {
+    this.folioNumber = folioNumber;
+  }
+
+  public String getSeries() {
+    return series;
+  }
+
+  public void setSeries(String series) {
+    this.series = series;
+  }
+
+  public Integer getInstallment() {
+    return installment;
+  }
+
+  public void setInstallment(Integer installment) {
+    this.installment = installment;
+  }
+
+  public Double getLastBalance() {
+    return lastBalance;
+  }
+
+  public void setLastBalance(Double lastBalance) {
+    this.lastBalance = lastBalance;
+  }
+
+  public Double getTotal() {
+    return total;
+  }
+
+  public void setTotal(Double total) {
+    this.total = total;
+  }
+
+  public String getCurrency() {
+    return currency;
+  }
+
+  public void setCurrency(String currency) {
+    this.currency = currency;
+  }
+
+  public Double getAmount() {
+    return amount;
+  }
+
+  public void setAmount(Double amount) {
+    this.amount = amount;
+  }
+
+  public List<PaymentSummaryTax> getTaxes() {
+    return taxes;
+  }
+
+  public void setTaxes(List<PaymentSummaryTax> taxes) {
+    this.taxes = taxes;
+  }
+
+  /** Prorated tax breakdown for the paid amount. */
+  @JsonIgnoreProperties(ignoreUnknown = true)
+  public static class PaymentSummaryTax {
+    private Double base;
+    private Double rate;
+    private String type;
+    private String factor;
+    private Boolean withholding;
+
+    public Double getBase() {
+      return base;
+    }
+
+    public void setBase(Double base) {
+      this.base = base;
+    }
+
+    public Double getRate() {
+      return rate;
+    }
+
+    public void setRate(Double rate) {
+      this.rate = rate;
+    }
+
+    public String getType() {
+      return type;
+    }
+
+    public void setType(String type) {
+      this.type = type;
+    }
+
+    public String getFactor() {
+      return factor;
+    }
+
+    public void setFactor(String factor) {
+      this.factor = factor;
+    }
+
+    public Boolean getWithholding() {
+      return withholding;
+    }
+
+    public void setWithholding(Boolean withholding) {
+      this.withholding = withholding;
+    }
+  }
+}
diff --git a/src/main/java/io/facturapi/resources/InvoicesResource.java b/src/main/java/io/facturapi/resources/InvoicesResource.java
index 6dac015..7cdbdd4 100644
--- a/src/main/java/io/facturapi/resources/InvoicesResource.java
+++ b/src/main/java/io/facturapi/resources/InvoicesResource.java
@@ -4,6 +4,7 @@
 import io.facturapi.http.FacturapiHttpClient;
 import io.facturapi.models.GenericResponse;
 import io.facturapi.models.Invoice;
+import io.facturapi.models.PaymentSummary;
 import io.facturapi.models.SearchResult;
 import java.io.InputStream;
 import java.util.Map;
@@ -52,6 +53,22 @@ public Invoice retrieve(String id) {
     return get("/invoices/" + id, null, Invoice.class);
   }
 
+  /**
+   * Gets the information needed to add this invoice as a related document in a
+   * payment complement (complemento de pago): the installment number according
+   * to the payment history, the previous balance, and the invoice tax breakdown
+   * prorated to the amount being paid.
+   *
+   * @param id Invoice id.
+   * @param amount Amount being paid, expressed in the invoice currency. Cannot
+   *        exceed the outstanding balance.
+   * @return Payment summary ready to be used as a related document.
+   * @see <a href="https://docs.facturapi.io/api#operation/getInvoicePaymentSummary">API reference</a>
+   */
+  public PaymentSummary paymentSummary(String id, double amount) {
+    return get("/invoices/" + id + "/payment-summary", Map.of("amount", amount), PaymentSummary.class);
+  }
+
   /**
    * Cancels an invoice.
    *
diff --git a/src/test/java/io/facturapi/FacturapiResourcesTest.java b/src/test/java/io/facturapi/FacturapiResourcesTest.java
index ef43828..e425076 100644
--- a/src/test/java/io/facturapi/FacturapiResourcesTest.java
+++ b/src/test/java/io/facturapi/FacturapiResourcesTest.java
@@ -46,6 +46,36 @@ void invoiceCreateUsesExpectedPath() {
     assertEquals("/v2/invoices?test=true", request.uri().getPath() + "?" + request.uri().getQuery());
   }
 
+  @Test
+  void invoicePaymentSummaryUsesExpectedPath() {
+    StubHttpClient httpClient = new StubHttpClient();
+    httpClient.enqueueJson(
+      200,
+      "{\"uuid\":\"6CF6CE33-1BD2-4F88-A443-33013C069169\",\"installment\":1,"
+        + "\"last_balance\":100,\"total\":100,\"currency\":\"MXN\",\"amount\":58,"
+        + "\"taxes\":[{\"base\":50,\"rate\":0.16,\"type\":\"IVA\",\"factor\":\"Tasa\",\"withholding\":false}]}"
+    );
+
+    Facturapi sdk = new Facturapi(
+      FacturapiConfig.builder("sk_test")
+        .httpClient(httpClient.client())
+        .build()
+    );
+
+    var response = sdk.invoices().paymentSummary("inv_1", 58.0);
+
+    assertEquals("6CF6CE33-1BD2-4F88-A443-33013C069169", response.getUuid());
+    assertEquals(1, response.getInstallment());
+    assertEquals(50.0, response.getTaxes().get(0).getBase());
+
+    var request = httpClient.requests().get(0);
+    assertEquals("GET", request.method());
+    assertEquals(
+      "/v2/invoices/inv_1/payment-summary?amount=58.0",
+      request.uri().getPath() + "?" + request.uri().getQuery()
+    );
+  }
+
   @Test
   void receiptsToInvoiceUsesExpectedPath() {
     StubHttpClient httpClient = new StubHttpClient();