Hi, maicart, Both your sentences are grammatically correct and the manager could say either one to a new customer. The first sentence is technically appropriate since it is likely that the manager would offer the "gift" after the customer opens a new account, but that phrasing might sound a bit pretentious and so the manager might use the less complicated and very "natural-sounding" second option.
OK, forero, the first choice is unlikely as it is "wordy" or "heavy," one might say, but I still think it is possible since one does not always hand over a "gift" or "bonus" at the moment of the transaction. One would present that gift after the customer has opened the account. So, maicart, go with "for opening," but know that the other option is grammatically possible.