Where did you get this frog? He is sick

. The amputation might be able to be healed with antibiotics. The light green spots appear to be a bacterial infection. I can't see the knees well, but he looks to either have injuries to them or infections. The red spots on his underside are not good

. This is very sad to see. Do you know if he was wild caught or captive bred?
You should first contact the seller and notify them of his condition and provide photos. I would request a refund of your money.
Next, he needs a hospital tank. A ten gallon tank is easiest. Use only plain paper towels as substrate and only spring water or dechlorinated tap water. If I were you I'd boil the water and allow it to cool to room temp before using it to be sure to not introduce any bacteria into his tank. Use only this water for his water bowl also. Rinse and change his water in his water bowl daily. Change his paper towels daily also.
You'll need a vet to prescribe medications to heal all of the wounds. He needs systemic antibiotics and topical for the wounds. I would not delay in contacting a vet. Quick treatment can save him.
Wash your hands very well, rinse thoroughly, then either wear powder-free gloves or rinse your hands with the spring water or dechlorinated water. Be sure there are no lotions or perfumes on your hands when handling him. Never use hand sanitizer, it can burn them.
For today, while waiting to hear from a vet, very gently apply ORIGINAL plain neosporin to the foot, his back on the green spots, the dark areas on the knees and the red spots on his underbelly. Be sure to cover the toe area well. Using a rolling motion helps to goop the neo on. You don't need a lot, but try to cover it.
I am sorry he was delivered to you in this way

. Poor little guy

.
To reduce his stress, keep his tank in a calm and quiet area. Cover the back and 2 sides of the tank with something such as green paper. This helps them to feel safe. You'll want to minimize his stress to keep him healing.