How suspension works

When the client's membership is on suspension, those invoices that fall into the suspension period cannot be adjusted.

And regarding your question, What if I suspend her for 3 weeks for example but then adjust her next payment? Does that work? - Yes, that would work. The next scheduled payment after those invoices falling under the suspension period can be adjusted. Please refer to the screenshot as an example of what I mean here.
 

 
Here's more about Suspension.
 
How suspension works:
 
When a client membership is suspended:
 
-  The client will not be able to book classes with membership during the suspension period.
-  No payment will be charged for membership during the suspension period.
-  The client will be able to make bookings with the membership after the suspension period is over/completed.
-  The system will not delay or shift the scheduled invoice further. The invoice which falls during the suspension period will not be processed/executed.
 
Please Note: While suspending a membership, the system will not move forward with the future payment schedule for the suspension period. The system will not process any invoice during the suspension period and cannot book any classes during the suspension period.