Hello, my name is ***** ***** it is my pleasure to assist you with your question today.
No the employer is not entitled to do this. The payment you have received is paid under your existing terms of employment, which you had worked under until resigning. You have specifically refused to accept the new terms and there is no evidence that you have accepted them in any way. The employer cannot pay you under your existing terms and argue that acceptance of that payment amounts to an acceptance of the new terms - had the amount in question been a separate payment which you were not entitled to under your existing contract they could pay it on the condition that it was linked to your acceptance of the new terms, but not otherwise. You may keep the payment, again make it clear that you refuse to accept the new terms and leave it at that - the clause in question is unlikely to be valid or enforceable
Something along the lines of "The payment I have received is due to me under the existing arrangements and is not consideration for the newly introduced terms, which I have explicitly refused to accept. I have made it clear before and will confirm again that I do not accept these new terms, specifically [briefly describe them] and that I do not consider them enforceable".If the employer wants to enforce them it would be for them to go to court and prove it was valid, which will be difficult in the circumstances