I have seen this answered elsewhere, but can’t remember where. Basically the seal of confession forbids the priest to do anything which would reveal the matter confessed. This definitely includes any requirement that the penitent reveal the sin.
He may require restitution or reparation, but only if it is possible without revealing the sin. For example, if stolen money cannot be returned without admitting the sin, he might require that one contribute the money to charity.
Sins are forgiven at the moment of absolution. The penance should be performed as soon as possible to avoid the possibility of forgetting. Deliberately failing to perform a penance is a new sin which must be confessed. If the penance has not been performed by the time of ones next confession this should be confessed, and explained if there is reasonable cause for the delay.