John Gill Commentary Exodus 34:20

"And the firstling of an ass thou shalt redeem with a lamb: and if thou wilt not redeem it, then thou shalt break its neck. All the first-born of thy sons thou shalt redeem. And none shall appear before me empty." — Exodus 34:20 (ASV)

Every firstling of an ass you shall redeem
This goes along with the former, (See Gill on Exodus 13:13):

and none shall appear before me empty ;
at the grand festivals, the passover, pentecost, and tabernacles, (See Gill on Exodus 13:15).