Bugfix : combination of payment surcharging and tax exempt sale was broken.  Result would be an unbalanced journal. Now fixed. Problem especially obvious / annoying with tillpay terminal integration as the surcharge is proscribed by the terminal and has to be applied somehow.

Root cause is c9 was assuming the surcharge was a 'inc gst' , so with txn tagged as tax free it was trying, incorrectly to subtract the tax component from it.