I remember alaa talking about her giving money to Salah, I think that might be how we know about it? If this happened, that means alaa knew that the father bought him into the biz (and I think this happened before he met chantal. He needed a job on paper for his visa, they might have been cracking down when kuwait "reopened" after covid, which was a few months before chantal met salah) AND chantal gave salah money. I was kind of under the impression that she thought she was also buying into the business herself - remember when she said she was going to create a line of soaps (irony)? Alaa might not be completely clear of the goings on, since he was smart enough to not invest in the business. The business was with Murad.
My favorite explanation for chantal's 9k is that Salah convinced her that the bride pays the dowry.