Real question: What is to stop someone with power/money from taking advantage (or just fooling) someone with UBI to sign away their future UBI income? For example, the UBI person wants to buy a car, but has nothing other than UBI, the car dealer says, "no problem, just sign here and all your UBI for the next 5 years will pay for the car. Not our problem if you don't have anywhere to park it or gas to run it". Historically, those with power are able to clip a little extra assets from someone who doesn't have the power. Why wouldn't the UBI just become a new baseline for almost-zero? I hope I'm phrasing this in a way that is understandable.
If the income really is guaranteed, competition in UBI-secured lending should eventually drive margins down to the point where the borrowing cost approaches the risk-free interest rate, so at least people are getting a reasonable deal on their loan.
Of course, it's still a serious social problem if people borrow against their future earnings to buy expensive things and then go years without enough money for food. It might not even be irrational, for example someone might borrow to pay for a life-saving surgery for a family member.