Do I have to record a rate lock extension fee refund from a bank?

That is a "rebate" of the fees you paid  then you just reduce the expense by the amount of the refund. 

Say you paid $100 but they gave you $20 back so now the expense was only $80.

A rate lock fee is not a deductible closing cost (unless it was considered deductible "points").  If you didn't deduct the cost, the refund has no tax consequences.  

If you did deduct the rate lock fee on a past tax return (whether as points, or by mistake) a refund of a past deduction is a "taxable recovery" and is listed under "other" or miscellaneous income.