Agreeance is a less-popular alternative to 'agreement'. They both mean the same thing, and both have been around for over 500 years, but despite continued reemergence, agreeance has never managed to overtake agreement in popularity.
Agreeance first appears in written English in 1536, while agreement appeared in 1398. However, at first agreement was only used to mean 'likeness' or 'conformity', with the meaning of 'consent' appearing much later, in 1479. Since the 1500s agreeance appears to have maintained a general recognizance as a cromulent if obscure word, with perhaps having particular popularity in Scotland (although sample sizes in published works are so small that this is mostly supposition).
The suffix -ance is entirely appropriate for this word, as is -ment; both are used to change verbs into nouns, and the technical differences that they had in Latin have long been eliminated as they traveled through French and on into English. Indeed, while French has also settled on agrément, the alternate agréance was used in Old French.
It is possible that the reoccurrence, and quite possibly, recoinage of agreeance comes from the recognition of a common noun formation pattern in English, hinted at by the fact that it usually occurs in the phrase "in agreeance". In English, words that take a definite or indefinite article tend to end in -ment ([the] government, [an] assignment, [an] assessment, [a] commitment, [the] punishment, [a] development); while words that do not need one tend to use -ance ([have] concordance, [meet] resistance, [have] tolerance, [come into] compliance,
[gain] acceptance, [make] allowance, [in] allegiance). English being English, there are many, many exceptions to this rule, e.g., equipment, alignment, ablatement, utterance, performance, remittance... and usually, agreement.