Is it permissible to make up the days that a pregnant woman missed fasting and to pay the expiation for breaking the fast later if she currently does not have the money?

There is no harm in a pregnant or breastfeeding woman breaking her fast, nor in delaying the make-up fast due to a valid excuse. The make-up fast remains a debt owed.

If it was possible to make up the fast but it was delayed until after another Ramadan without a valid excuse, then the minor expiation (feeding one needy person for each day) becomes obligatory. There is no harm in delaying it due to lack of funds.

If she is unable to make up the fast due to a valid excuse, then there is nothing upon her.