You can say both with or without the för. There might be a very slight nuance in meaning. Your sentence would be Visst, jag kan betala för er mat.
You already changed the auxiliary verb kunna into kan so the next verb betala must not be changed. It would be like saying he can pays in English instead of he can pay.
You also want the possessive pronoun er (your). Ni is a personal pronoun which means ’you, you guys, y’all’.
I want to reply to Lundgrens post. In Finland we use both languages: Finnish and swedish. And we use alot "Ni" when we mean "you" to elderly people or higher people in our hiararchy, example in army. I dont know do they use it in other world. But in Finland it is more than acceptable to use when speaking to elderly or as i example in army or higher person in hiararchy.
"offer" does not mean "bjuder", it means "erbjuder". The prefix changes the meaning. "Bjuder" translates into "pay" when you go to the restaurant. "Bjuder" is also what you do in your home.
Jag bjuder dig hem till mig. I invite you to my home.
Jag bjuder på smörgåsar. I serve sandwiches. NB it is "bjuder på" for "serve".
In English the word "tale" used to mean count/give an account of, which is done to facilitate paying. Probably as "tally" in "Come, Mister tally man, tally me banana". Related to English "tell" and the "tale" that you tell. Counting, paying, and speaking (giving an account of something or recounting a tale) all seem related in several languages.