fix: Transfer excess back and reset allowance (#28)

Author: miguelmtzinf

src/contracts/BaseParaSwapBuyAdapter.sol

@@ -38,6 +38,7 @@ abstract contract BaseParaSwapBuyAdapter is BaseParaSwapAdapter {
    * @param maxAmountToSwap Max amount to be swapped
    * @param amountToReceive Amount to be received from the swap
    * @return amountSold The amount sold during the swap
+   * @return amountBought The amount bought during the swap
    */
   function _buyOnParaSwap(
     uint256 toAmountOffset,
@@ -46,7 +47,7 @@ abstract contract BaseParaSwapBuyAdapter is BaseParaSwapAdapter {
     IERC20Detailed assetToSwapTo,
     uint256 maxAmountToSwap,
     uint256 amountToReceive
-  ) internal returns (uint256 amountSold) {
+  ) internal returns (uint256 amountSold, uint256 amountBought) {
     (bytes memory buyCalldata, IParaSwapAugustus augustus) = abi.decode(
       paraswapData,
       (bytes, IParaSwapAugustus)
@@ -73,7 +74,6 @@ abstract contract BaseParaSwapBuyAdapter is BaseParaSwapAdapter {
     uint256 balanceBeforeAssetTo = assetToSwapTo.balanceOf(address(this));
 
     address tokenTransferProxy = augustus.getTokenTransferProxy();
-    assetToSwapFrom.safeApprove(tokenTransferProxy, 0);
     assetToSwapFrom.safeApprove(tokenTransferProxy, maxAmountToSwap);
 
     if (toAmountOffset != 0) {
@@ -98,13 +98,15 @@ abstract contract BaseParaSwapBuyAdapter is BaseParaSwapAdapter {
         revert(0, returndatasize())
       }
     }
+    // Reset allowance
+    assetToSwapFrom.safeApprove(tokenTransferProxy, 0);
 
     uint256 balanceAfterAssetFrom = assetToSwapFrom.balanceOf(address(this));
     amountSold = balanceBeforeAssetFrom - balanceAfterAssetFrom;
     require(amountSold <= maxAmountToSwap, 'WRONG_BALANCE_AFTER_SWAP');
-    uint256 amountReceived = assetToSwapTo.balanceOf(address(this)) - balanceBeforeAssetTo;
-    require(amountReceived >= amountToReceive, 'INSUFFICIENT_AMOUNT_RECEIVED');
+    amountBought = assetToSwapTo.balanceOf(address(this)) - balanceBeforeAssetTo;
+    require(amountBought >= amountToReceive, 'INSUFFICIENT_AMOUNT_RECEIVED');
 
-    emit Bought(address(assetToSwapFrom), address(assetToSwapTo), amountSold, amountReceived);
+    emit Bought(address(assetToSwapFrom), address(assetToSwapTo), amountSold, amountBought);
   }
 }

src/contracts/ParaSwapDebtSwapAdapter.sol

@@ -217,7 +217,7 @@ abstract contract ParaSwapDebtSwapAdapter is
     IERC20Detailed newDebtAsset,
     uint256 newDebtAmount
   ) internal returns (uint256) {
-    uint256 amountSold = _buyOnParaSwap(
+    (uint256 amountSold, uint256 amountBought) = _buyOnParaSwap(
       swapParams.offset,
       swapParams.paraswapData,
       newDebtAsset,
@@ -234,6 +234,12 @@ abstract contract ParaSwapDebtSwapAdapter is
       swapParams.debtRateMode,
       swapParams.user
     );
+
+    //transfer excess of old debt asset back to the user, if any
+    uint256 debtAssetExcess = amountBought - swapParams.debtRepayAmount;
+    if (debtAssetExcess > 0) {
+      IERC20WithPermit(swapParams.debtAsset).safeTransfer(swapParams.user, debtAssetExcess);
+    }
     return amountSold;
   }