What is the ruling on taking a loan from an Islamic finance company to purchase goods in installments with an increase, then selling them back to the same seller for cash to obtain liquidity, given that the purpose of the loan is to cover marriage expenses?

If the company buys the goods, takes possession and responsibility for them, and then sells them to you in a separate contract, there is no harm in that. Your selling the goods to the same entity from whom the company bought them does not affect the validity of the transaction. However, an agreement with the seller to return the goods to him is a forbidden stratagem and a sinful transaction. This transaction would be invalid on your part, not on the company's part. Therefore, it is not permissible to agree with the seller to return the goods. Instead, you should look for someone to buy them from you after the company buys them from you. It is preferable that the buyer not be the original seller, to avoid the suspicion of 'inah (buy-back) sale.