Hilchos Bikkurim 11:6

The mitzvah of pidyon haben applies in all times and places. A firstborn son is redeemed for five sela as per Numbers 18:16. Redemption may be made using silver or something worth silver, such as movable property that has a financial value like shekels. One may not redeem a firstborn son using real estate or servants, nor with promissory notes as they have no inherent value. If someone redeems a firstborn son using one of these, the son is not redeemed.

Hilchos Bikkurim 11:7

If a father writes to a kohein that he commits to giving him five sela, he is so obligated, but the son is not redeemed. If he gives him an object that isn’t worth five sela in the market, but the kohein accepts it as if it were five sela, the son is redeemed. If the father divides five sela among ten different kohanim, whether he does so all at once or in succession, he has fulfilled his obligation (after the fact).