I agree with Tanaka-san's answer. Or you can also say そのような大切なものはとても受け取れません。お気持ちだけいただきます (or いただいておきます)。 You don't need to say "Please take it back" after that. As you know, Japanese people tend to avoid saying everything or saying something directly. If you must say "take it back" to him, you can say お手元で大切になさってください or something. By the way, for old people, it's a pleasure / happiness to give their important things to young people who they like. So, if you receive his favor, I think he would be happier.

Hmm, I think this is a very good sentence! Suppose that, that person has become kind of like my best friend but is very much older than me, perhaps double than me! If possible, can you tell me if it would also sound nice if I added a sentence like "Please take it back" AFTER the sentence you wrote? Thanks! ありがとうございました。(^_^)

お気持ちは大変うれしいのですが、そのような大切なものは受け取れません。ありがとうございます。 how about this? it is better to explain me the relation with u and that person.