If by psychological debt you mean a debt of honour or obligation to Mr Gardiner for looking after his daughter then I agree this would be a debt that Mr Bennet (and no other parent faced with the same situation) could ever repay. However, I believe Mr Gardiner was more capable of managing the arrangements, including financially, and the Bennet's themselves appear to agree.

Firstly, Mr Bennet, Jane and Elizabeth discuss Mr Gardiner's payment to Wickham of what must have been 10,000 pounds and only Elizabeth refers fleetingly to the possibility that this sum of money might impact on the financial security of the Gardiners. Later, in Chapter 49, Mrs Bennet says "If he (Mr Gardiner) had not had a family of his own, I and my children must have had all his money, you know." This suggests to me that the Gardiners are likely to have had greater access to ready money (cash) than the Bennets.

Secondly, in Chapter 52, through Mrs Gardiner's letter, we learn that Mr Darcy is of the opinion that the Bennet's were "not imagined to be very rich". This information further adds to our knowledge that while the Bennet's had an income from property, and the interest from their mother's money they have very little else by way of income. And this appears to be understood outside the immediate family as Mr Darcy, while he may not be aware of Elizabeth's specific financial situation, at least understands it generally.