L1 A/B without US payroll?

Hello. My close relative's employer wants him to work on and off in the USA while on UK payroll. They are in the process to get him a L1A or L1B for their USA business unit while keeping him on UK payroll. Is that even possible?

Would I need to have some minimum US pay in order to retain a L1 visa?

Assuming this works, would I be liable for US taxes for my UK payroll income as a US tax resident?

Thanks in advance.