Promote to Sage 300


Hi we are having an issue with our Sage CRM and promoting to Sage 300.

We haven't used this function before resulting in a lot of duplicate customers being in CRM. This is because we have them entered as prospects and then we sent them up as actual customers if the time comes to that. We set them up as customers in Sage 300 which then results in duplicate companies (one a prospect and one sage customer). 

When in the prospect we click the Promote to Sage 300 tab it opens up saying select sage 300 company - it shows our company name and has a spot for Sage 300 Customer Number/Sage 300 Vendor Number. We type in what should be the customer number and hit the "Promote to Customer" button. We then get the following message: #### already exists in Sage 300 Company.

Can confirm number is not used and company isn't in Sage 300. Thought this might be a permission error so we tried under an admin login and received the same message. 

Any ideas?