Captures currency-linked transactions late

I recently made an oline purchase in pounds and got an sms for R1861 and some change. The transaction was processed a day o so later and came to R1949, clearly an exchange rate issue.

I went into the branch here at Fourways, and 40mins later, still, nobody could tell me why. I heard about minimum costs, and merchant fees, but no actual answer. All I really want is the difference, or a reasonable answer as to WHAT exactly the "extra" charge was for.