server: fix negative fees

server/app/com/xsn/explorer/models/TransactionInfo.scala

@@ -13,7 +13,7 @@ case class TransactionInfo(
     received: BigDecimal,
     height: Height
 ) {
-  def fee: BigDecimal = sent - received
+  def fee: BigDecimal = (sent - received).max(BigDecimal(0))
 }
 
 object TransactionInfo {

server/test/com/xsn/explorer/models/TransactionInfoSpec.scala

@@ -0,0 +1,38 @@
+package com.xsn.explorer.models
+
+import com.xsn.explorer.helpers.DataGenerator
+import com.xsn.explorer.models.values._
+import org.scalatest.matchers.must.Matchers
+import org.scalatest.wordspec.AnyWordSpec
+
+class TransactionInfoSpec extends AnyWordSpec with Matchers with DataGenerator {
+  "fee" should {
+    "calculate the paid fee" in {
+      val transactionInfo = TransactionInfo(
+        randomTransactionId,
+        randomBlockhash,
+        0L,
+        Size(0),
+        BigDecimal(125),
+        BigDecimal(25),
+        Height(0)
+      )
+
+      transactionInfo.fee mustBe BigDecimal(100)
+    }
+
+    "return 0 when result would be negative" in {
+      val transactionInfo = TransactionInfo(
+        randomTransactionId,
+        randomBlockhash,
+        0L,
+        Size(0),
+        BigDecimal(25),
+        BigDecimal(125),
+        Height(0)
+      )
+
+      transactionInfo.fee mustBe BigDecimal(0)
+    }
+  }
+}