Q: The shul auctioned off the chazak aliya, ending Sefer Bereishis. I bought the aliya, but felt sick during leining, and had to leave shul, so that someone else got the aliya. Must I pay?
A: The poskim dispute whether selling aliyos is considered a valid sale/ rental or merely a pledge (see Mishneh Halachos 5:304; Hayashar V’Hatov vol. 14 p. 86, vol. 21 p. 143).
If it is a sale/rental, your personal oness would not exempt you from your obligation. However, if it is a pledge, you clearly pledged with the expectation that you be able to benefit from the aliya.
Nonetheless, it is common in many places to buy an aliya and honor someone else, such as the Rov of the shul or a relative. Although you had to leave, you could have instructed the gabbai to honor someone else, and thereby benefit from the aliya. Therefore, you are liable to the shul for your pledge.
Nonetheless, if the person who got the aliya donated a certain sum, and certainly if he bought the aliya, you can deduct that amount from your pledge (Magen Avraham 154:23).
