I have an expense which is reimbursable from the customer. I have added the expense in the invoice and the amount is fully paid. Still, the expense is shown as (-) in PNL

Hello vulnerable-innoc, 

Welcome to the Community. QuickBooks is a powerful program that helps you to manage your accounting with ease. I'll be glad to share more info so you're on the right track with your work. 

Recording your expenses the right way is essential for ensuring your books remain accurate. That said, I recommend consulting an accounting specialist for expert advice. This ensures your entries are recorded in a way that allows for you to navigate the program with peace of mind. You can get in touch with an accounting professional using this link here. Other accounting professionals are able to see this post in the Community and this will allow them to chime in. 

Let me know if this info helps. I'm here to steer you in the right direction.