بسم الله الرحمن الرحيم
(a) Rahman is among those Beautiful Names that are specified with Allah. (See Fatawa Usmani 1/50, pub. Naimia). So it is not correct to keep this name without adding Abd or Amat. Thus either you keep the name of your daughter only Safiya and do not include Rahman or keep her name Safiya Amatur Rahman. Amatur Rahman means Allah’s slave girl while Safiya is the name of the holy wife of the Messenger of Allah (may peace and blessings of Allah be upon him).
(b) However, if you keep Safiya Rahman and it is based upon genitive construction that Safiya is mudaf towards Rahman then this name too will be correct. In this case it will mean Allah’s sincere friend. As Safiya also means sincere friend. (See Misbahul Lughat, p. 452)
So as per the explanation made above regarding the name it is allowable to keep the name Safiya Rahman.
Allah knows Best!
Darul Uloom Deoband