Merge is putting a formula (SUM) into Pages. In Numbers, convert to values (Copy, then Edit > Paste Values).
That will replace the formulas, so do it on a spare copy of your Numbers document (or a spare copy of the table).
Or, add another column to your Numbers table and Paste Values into that. Use that column as your merge source.
I doubt if there is anything wrong with your Numbers document.
I've tried to duplicate you problem and have been completely unsuccessful. Mail Merge will accept a SUM in a Numbers cell which is formatted as currency and enter it into the Pages document as it displays in Numbers. this actually worked for me with currency values entered directly and with ones derived from a calculation. It worked when merging into regular text in Pages and when merging into a cell in a Pages table. The only way I could get it to display as an integer, without the dollar sign and the decimal part was when I merged into a cell of a table that was explicitly formatted as a number with no decimal places.
So, I think it is a problem in your Pages document. What happens if you make a new, blank word processing document, link it to your Numbers document and insert, into the main text body, nothing but a merge field to your SUM field in the Numbers document? Does a merge still change the formatting?