All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Murabaha (a sales contract where the buyer and seller agree on the markup (profit) or "cost-plus" price for the item(s) being sold.) contract takes place when the seller delivers the sold item to the buyer. If that item was bought from overseas, handing over and assigning the bills of lading to the buyer concludes the sales contract. By doing this, the contract is concluded, the possession of the sold item is transferred to the buyer and the latter becomes liable for its price.
Therefore, financing the customs duty can`t be considered a new sales contract because the latter has already been concluded, and the financing, in this situation, leads to committing Riba (Usury/interest). Messenger of Allah (PBUH) said, "What is lawful is clear and what is unlawful is clear, but between them are certain doubtful things which many people do not know. So he who guards against doubtful things keeps his religion and his honour blameless. But he who falls into doubtful things falls into that which is unlawful, just as a shepherd who grazes his cattle in the vicinity of a pasture declared prohibited (by the king); he is likely to stray into the pasture. Mind you, every king has a protected pasture and Allah's involved limits is that which He has declared unlawful. Verily, there is a piece of flesh in the body, if it is healthy, the whole body is healthy, and if it is corrupt, the whole body is corrupt. Verily, it is the heart." [Al-Bukhari and Muslim].
In order for the above financing to become valid, the bank must delay concluding the sales contract until after the customs are paid for, and make them part of the expenses. And Allah knows best.