Hi Nataliia, saying "I will pay you back" doesn't always have a negative connotation. Only if you use it to mean "I will get revenge". But it's pretty common to say "Can I borrow $5? I will pay you back." If it's not money you are discussing, but an exchange of favors, then you can say "I'll return the favor" or "I'll make it up to you". Or even "I will owe you one", meaning, I will owe you a favor if you do this for me. So you could say "Can you baby sit for me? I will make it up to you". Or "Can you wash the dishes for me? I will return the favor". I hope that helps!
~ Helena ~