If specific restitution, with or without a sum of money, will be substantially as effective as restitution in money in putting the party claiming restitution in the position he was in before rendering any performance, the other party can discharge his duty by tendering such restitution before suit is brought and keeping his tender good.
tendering would mean offering to make the designated restitution before a lawsuit is filed, and then actually making that restitution. the tender is the offer, so keeping that tender good means actually following through on the offer.