It is a common practice to issue a Conditional Swift MT 199 which is based on the premises that the amount to be remitted is still not finalised because (in this case) ROB figures may not be finalised in time.
Therefore, it is the usual practise for Buyers to remit the excess funds using the Conditional SWIFT MT 199(which states the conditions for fund release) and upon finalising the amount (which to be agreed between buyers and sellers on the day of delivery), the SWIFT MT 103 is being send out by Buyers bank to release the agreed amount strictly based on the conditions set out on the Conditional SWIF MT 199
Any excess money will be returned back to Buyers