Promise of Double Reward in Return for Giving Up Now

Shalom.
A teacher distributed candies to his class, and one candy was missing for one of the children. The teacher asked one of the children to give his candy to the child who did not receive one and was crying a lot, on condition that the next day he would receive two candies. Is this permitted? And if not, what can be done to appease the child in a permitted way?

It is forbidden. The solution is that he should tell the child that this involves ribit (interest) and that he is not allowed to give the additional candy, and afterwards he should ask another teacher to, of his own accord, give the child an extra candy as compensation (and as a mark of heroism for fulfilling the mitzvah of the laws of ribit).

Shulchan Aruch, Yoreh De’ah 168:21 explains that the one who accepts responsibility for the loan is considered the borrower (and similarly in siman 170 regarding a guarantor), and here the teacher accepted upon himself the responsibility and is forbidden to give interest on the loan. And as explained in 161:1, even less than a perutah’s worth is forbidden as ribit. Nevertheless, after they have spoken with the child and explained that there is ribit here and that, because of the fixed agreement made between them, he will not receive it, another teacher may appease him by giving the candy, for the Shach 160:18 rules like the early authorities that, in principle, it is permitted for the borrower to appease another person to give interest on his behalf. However, care must be taken that the lender not stipulate with the borrower that another person should give interest on his behalf (and therefore one must first tell the child that, by virtue of the fixed agreement, he will not receive it, and only afterwards it is considered a new gift), and also that the one giving the interest not later take it back from the borrower.