Perhaps you have already heard that Russians have a very weird phrase to show uncertainty: да нет, наверное, and it is translated as "yes, no, maybe". Actually, people use it just to say maybe — может быть.
- If you need time to understand if you want to go or not, the following phrases can come in handy: я еще не решил(а), which means "a haven't decided yet" or я не уверен(a) — "Ι am not sure".
- If you want to say that you will think about it, use я не могу дать вам ответ сейчас because its direct translation is "I can't give you an answer now". A very common phrase is я подумаю над этим and it is for "I will think about it".
Now you know how to reject an offer in Russian in a polite way and to ask for some time to think about it. All these phrases are very typical and quite universal so you can use them in different situations when you need to refuse or reject someone's offer.