diff --git a/src/CustomerData.txt b/src/CustomerData.txt
index 3ac070f..782579a 100644
--- a/src/CustomerData.txt
+++ b/src/CustomerData.txt
@@ -1,6 +1 @@
-1234=example
-ID=1234
-forename=Max
-Surname=Mustermann
-password=example
-balance=0
+balance=500
\ No newline at end of file
diff --git a/src/TempData.txt b/src/TempData.txt
new file mode 100644
index 0000000..f81d5a8
--- /dev/null
+++ b/src/TempData.txt
@@ -0,0 +1,5 @@
+1234=example
+ID=1234
+forename=Max
+Surname=Mustermann
+balance=0
diff --git a/src/a.exe b/src/a.exe
new file mode 100644
index 0000000..25d0c58
Binary files /dev/null and b/src/a.exe differ
diff --git a/src/updateCustomerAccountBalance.c b/src/updateCustomerAccountBalance.c
new file mode 100644
index 0000000..48ad5e8
--- /dev/null
+++ b/src/updateCustomerAccountBalance.c
@@ -0,0 +1,95 @@
+#include "updateCustomerAccountBalance.h"
+#include "currentCustomerAccountBalance.c"
+
+void deleteLineFromFile(int line){
+    FILE *fptr1, *fptr2;
+    char file1[] ="CustomerData.txt";
+    char file2[] ="TempData.txt";
+    char curr;
+    int del=line, line_number = 0;
+    fptr1 = fopen(file1,"r");
+    fptr2 = fopen(file2, "w");
+    curr = getc(fptr1);
+    if(curr!=EOF) {line_number =1;}
+    while(1){
+      if(del != line_number){
+        putc(curr, fptr2);
+        curr = getc(fptr1);
+        if(curr =='\n') line_number++;
+        if(curr == EOF) break;
+      }else{
+        
+      }
+
+    }
+    fclose(fptr1);
+    remove(file1);
+    fclose(fptr2);
+    rename(file2,"CustomerData.txt");
+    
+}
+
+void replaceBalanceInString(float replacementBalance, int currentLine) {
+    deleteLineFromFile(currentLine-1);
+    printf("deleting from line %i. balance will be replaced with %f", currentLine, replacementBalance);
+    char newBalanceLine[MAX_LENGTH] = "balance=";
+    char balance_as_string[MAX_LENGTH];
+    sprintf(balance_as_string, "%g", replacementBalance); //converts replacement balance to string
+    strcat(newBalanceLine, balance_as_string);
+    printf(newBalanceLine);
+
+    char buffer[MAX_LENGTH];
+    FILE *file = fopen("CustomerData.txt", "w"); 
+
+    for(int i=0;i<currentLine;i++){
+        fgets(buffer, MAX_LENGTH, file);
+    }
+    fprintf(file, newBalanceLine);
+    fclose(file);
+}
+
+bool updateAvailableAccountBalance(int user_id, float changeInBalance){
+
+    bool keep_reading = true;
+    float availableBalance = 0;
+
+    char buffer[MAX_LENGTH];
+    char stringID[MAX_LENGTH] = "ID=";
+    char user_id_as_string[MAX_LENGTH];
+    char balance_String[MAX_LENGTH];
+    int currentLine=0;
+
+    sprintf(user_id_as_string, "%d", user_id); // converts user_id to string
+    strcat(stringID, user_id_as_string);
+
+    FILE *file = fopen("CustomerData.txt", "r+"); 
+
+    while(keep_reading) {
+        fgets(buffer, MAX_LENGTH, file);
+        currentLine++;
+        if (feof(file)) {
+            keep_reading = false;
+        }
+        else if(strstr(buffer, stringID)) { //found the customer
+            fgets(buffer, MAX_LENGTH, file);
+            fgets(buffer, MAX_LENGTH, file);
+            fgets(buffer, MAX_LENGTH, file);
+            fgets(buffer, MAX_LENGTH, file);
+            strcpy(balance_String, buffer);
+            currentLine+=4;
+            availableBalance = fetchBalanceFromBalanceString(balance_String);
+            replaceBalanceInString(availableBalance+changeInBalance,currentLine);
+            keep_reading = false;
+        }
+
+    }
+    fclose(file);
+}
+
+int main(){
+
+    updateAvailableAccountBalance(1234,500);
+    return 0;
+
+}
+
diff --git a/src/updateCustomerAccountBalance.h b/src/updateCustomerAccountBalance.h
new file mode 100644
index 0000000..a5a4ce3
--- /dev/null
+++ b/src/updateCustomerAccountBalance.h
@@ -0,0 +1,9 @@
+#include <stdio.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define MAX_LENGTH 100
+bool updateAvailableAccountBalance(int user_id, float changeInBalance);
+void deleteLineFromFile(int lineNumber);
+void replaceBalanceInString(float replacementBalance, int currentLine);
\ No newline at end of file