The Torah forbids charging or paying interest on loans. If somebody borrows money, he may not pay the lender anything beyond the sum that was borrowed. The Hesed La’alafim (Rav Eliezer Papo, 1786-1827) writes (listen to audio recording for precise citation) that this applies even to the honor and privilege of receiving an Aliya to the Torah. If a person borrows money from somebody else in his congregation, he may not purchase an Aliya for his creditor as an expression of gratitude for the loan. This is permissible only if it is a favor he would normally have done for the lender, even if he had not lent him money. But if he would not normally purchase an Aliya for this fellow, and he does so only to express gratitude for the loan, this is forbidden, because the lender would then be receiving not only the sum of money which he lent, but also the honor and privilege of reciting the Berachot over the Sefer Torah, which would thus constitute Ribbit (interest).
Summary: A person may not purchase an Aliya in the synagogue for his creditor to express gratitude for the loan, unless he would have done him this favor regardless of the loan.