Programs with fewer than 20 trainees do not automatically receive additional compensation. If your program asks you to take on the administrative chief role without extra pay, you can reasonably say no, without fear of retaliation. 

Perhaps your PD would consider paying you out of departmental funds. This is an option under paragraph 16 of the collective bargaining agreement.

CHIEF RESIDENT SUPPLEMENTAL SALARY

A House Officer who is appointed as a Chief Resident consistent with SPG 201.10 and the Employer's established guidelines, which may be amended by the Employer from time to time, shall receive additional salary during the appointment period. The amount of a Chief Resident stipend shall not be less than $4000 per appointment year during the term of this agreement. In the event that more House Officers are offered and accept Chief Resident appointments in a particular program than the number of Chief Resident stipends allocated to that program pursuant to the Employer's guidelines, the funds available will be evenly divided among all Chief Resident appointees in the program, resulting in each Chief Resident receiving less than the full stipend amount specified in this paragraph; or in the alternative, the program may, at its discretion, provide additional funding through its department short code, so that each Chief Resident receives $4000.