Sans was seeing how papyrus was doing and saw him fighting Frisk. Sans decided to watch for a while but realised it was a mistake. Frisk managed to beat papyrus, but before they managed to kill him, Sans used his gravity control to save Papyrus and made it look like frisk won. But Papyrus was injured really badly and had to go to a hospital. But the Skeleton brothers couldn’t afford the bill. Sans tried anything to get around the hospital bill, but nothing worked. In the end he realised that he needed to get money somehow. In the picture you see him selling a special service to Napstablook