Bug fix: A line would get removed after the replaced line.

src/CustomerData.txt

@@ -3,7 +3,7 @@ ID=1234
 forename=Max
 Surname=Mustermann
 password=example
-balance=2000
+balance=5000
 
 1327=example
 ID=1327

src/updateCustomerAccountBalance.c

@@ -20,6 +20,7 @@ void replaceLineInFile(const char* file_name, int line, const char* new_line){
     while (fgets(current_string, sizeof(current_string), file) != NULL) {
         if (current_line == line) {
             fprintf(temp_file, "%s", new_line);
+            fputs("\n", temp_file);
         } else {
             fprintf(temp_file, "%s", current_string);
         }
@@ -81,3 +82,9 @@ bool updateAvailableAccountBalance(int user_id, float changeInBalance, bool posi
     float newBalance = (positive) ? availableBalance+changeInBalance : availableBalance-changeInBalance;
     replaceBalanceInString(newBalance,currentLine);
 }
+int main(){
+
+    updateAvailableAccountBalance(1234,500,true);
+
+    return 0;
+}