Based on the full image you've provided, you can see that it's not the full word - it's continued on the next line. Building on what /u/Rhodis suggested, the full word would be "juramentis", part of the phrase "instrumentis juramentis interpositis" - "the admitted/pledged instrument of oath", or something like that, presumably talking about another legal document. Does that help at all?

It's iuramentis, the 'tis' continues on the following line. So, ...publicis instrumentis iuramentis interpositis factis renunciationibus.... It's in Niermeyer's Latin as juramentis. The first part is standard notarial language as of 13th c. for referring to sworn legal documents.

I read it as "iuramen", meaning swearing, as in an oath.

This doesn't seem to be a common Latin word, I could only find it in the DMLBS.

IMO 2 things would help immensely. A higher quality image of the excerpt and a reasonable quality image of the whole page it's taken from.

It looks like "unamen" (which could be a contraction of "una men") to me but I can see there are some light pen traces but they're not clear enough through the compression artefacts of the low quality JPEG.

The Vatican library has high quality images for a lot of works now, is the bull available there?