AW StoreCredit

1. Open skin/frontend/base/default/aw_storecredit/js/aw_storecredit_payment.js

2. Find the following lines:

   if (!this.isStorecreditSubstracted && this.storecreditCheckbox.checked) {
       this.quoteBaseGrandTotal -= this.baseBalance;
       this.isStorecreditSubstracted = true;
   }
   
   if (this.isStorecreditSubstracted && !this.storecreditCheckbox.checked) {
       this.quoteBaseGrandTotal += this.baseBalance;
       this.isStorecreditSubstracted = false;
   }
   

3. Replace them with:

   if (!this.isStorecreditSubstracted && this.storecreditCheckbox.checked) {
       this.quoteBaseGrandTotal -= this.baseBalance;
       this.baseStorecreditAmountUsed = this.baseBalance;
       this.isStorecreditSubstracted = true;
   }
   
   if (this.isStorecreditSubstracted && !this.storecreditCheckbox.checked) {
       this.quoteBaseGrandTotal += this.baseBalance;
       this.baseStorecreditAmountUsed = 0;
       this.isStorecreditSubstracted = false;
   }
   

4. Save the file and try to use credits at firecheckout page.