Richards, who has excelled for the Pilgrims since arriving on loan in September, is being sent on loan to League of Ireland side Sligo Rovers instead.
Richards scored once in 21 appearances for the Pilgrims and moves on with the very best wishes of everybody at The Jakemans Stadium.
Robbins, meanwhile, has joined Vanarama National League North rivals Corby Town on a month’s loan. He has four goals to his credit for the Pilgrims following his move from Stamford AFC in October and is heading to Steel Park in pursuit of more regular first team action.
He could make his Corby debut against Gainsborough Trinity on Saturday.
Goalkeeper Jason Alexander has left The Jakemans Stadium by mutual consent after cancelling his contract with the Pilgrims.
United full-back Conor Green, meanwhile, has penned dual registration forms with Stamford and could feature for the Daniels in their weekend clash against Whitby Town at Borderville.
On the incomings front, United boss Dennis Greene has registered former Scarborough Athletic and Brighouse Town left-back Tommy Cooney ahead of Saturday’s home clash against Hednesford Town.
The 19-year-old hails from Galway in Ireland and has impressed in training with the Pilgrims.