Is Boeing paying for the engines or the airlines, as is the case usually with bigger planes? I remember that Boeing said CFM keeps production rates to compensate for the troubles in the beginning. Now, if Boeing has to pay 100 engines a month, that's quite some cash.
If its the airlines, I don't see them paying either.
Most NB customers buy the air frame and engine as a package from A or B, in contrast to what was fashionable with WB purchases for a while, where bigger customers negotiated air frames and engines separately.
This changed following RR, PW and CFM engines issues, in most cases driven by the finance industry, keen to avoid the situation where air frame and engine delivery is not synchronised, resulting in possible air frame penalty payments due to late delivery of engines.
Whether MAX customers have purchased a complete aircraft package through Boeing, or air frame and engines separately, engine related milestone payments will still fall due. If purchased as a package, these are passed on by Boeing to the customer, or if contracted separately, invoiced directly by GE to the customer.
As it's Boeing's choice to keep the MAX production lines open for aircraft it's customers cannot take delivery of or operate, engine payment milestones are being met by Boeing. Other than for customers that fund aircraft acquisitions from petty cash, the majority have a financiers involved, who would definitely not authorise milestone payments.
CFM and PW require minimum 2 year forward commitments from air frame OEM's, one of the reasons why A220 production is currently constrained, and MRJ too.
The temporary reduction in MAX production creates the opportunity for Airbus to increase production of CFM powered versions of the A32NEO family, and allocate some PW engine production capacity previously committed to the A32NEO, to instead build A220 spec engines.