We have shipped a SaaS product to the market very recently and one of our bigger customers have shown interest in investing in our company. They have requested a "code review" to evaluate if the product can scale easily. What's your thought on this? Is this a standard procedure before investing in a software company? If so, how can we do so without endangering our IP?
Firstly I don't think biz dev deals should be combined with financing. If the deal goes through, now you have a potential conflict of interest because a key customer is now an investor who has some say about future direction of the product/company
Better to create an SLA level offering and charge them more for any custom development.
Your question was how to do this without endangering your IP. This is not legal advice for your situation, but in this situation a letter of intent and/or also a NNN (non-compete, non-circumvent, non-disclosure agreement) agreement would be appropriate.