We are commanded to lend to a poor Jew and it is forbidden to harass him regarding repayment, as it says "If you lend money to My people, to the poor among you, you shall not be demanding to him",¹ and it says "[He shall not harass his friend...] you may harass the stranger... [you shall not harden your heart or close your hand to your poor brother; you shall open your hand to him] and you shall give him a pledge [according to his needs]".^2,a

A debtor must not evade paying his debts, as it says "Do not say to your friend `Go and come back'".³ All his property is liable for the debt; if the loan was made on a note, the creditor can collect, if necessary, even from real estate that the debtor sells after the loan. If he is bankrupt he has no further liability; but the post-Talmudic sages decreed that he must take an oath that he has nothing but necessary household goods.^b

It is forbidden to take collateral for a loan forcibly, as it says "[If you have any demand against your friend you shall not come into his house to take collateral;] you shall stand outside [and the man against whom you have a demand shall bring the collateral out to you]".⁴ If the debtor is poor and the collateral was not taken at the time of the loan we are commanded to return it to him whenever he needs it and we are forbidden to keep it from him, as it says "[And if he is a poor man] you shall not sleep with his collateral; you shall return the collateral to him by sunset",⁵ and it says "[If you take your friend's garment as collateral] you shall return it to him by sunset".⁶ It is forbidden to take as collateral utensils that are used to prepare food, as it says "[He shall not take a mill as collateral] for he is taking life as collateral".⁷ It is forbidden to take collateral from a widow, as it says "You shall not take the garment of a widow as collateral".^8,c

It is forbidden to lend money to a Jew at interest, as it says "[And if your brother becomes poor...] you shall not take interest or increase from him... you shall not give him your money at interest and you shall not give him your food at increase".⁹ It is forbidden to borrow money at interest, as it says "You shall not take from [or: give to] your brother interest [of money, interest of food, interest of anything taken for interest]".^l0 It is forbidden to participate in a loan that involves interest (as guarantor, scribe or witness), as it says "[If you lend money to My people...] you shall not put interest on him".¹ It is permitted to lend to or borrow from a non-Jew at interest, as it says "You shall take interest from the stranger [but from your brother you shall not take interest]"¹¹; but the sages forbade doing this except as necessary to support oneself.^d

Sources:
1. Ex. 22:24	a. 1:1-2
2. Deut. 15:2-3,7-8	b. 1:3; 2:1-2; 11:4; 18:1-2
3. Proverbs 3:28	c. 3:1-2,4-5
4. Deut. 24:10-11	d. 4:2; 5:1-2
5. Deut. 24:12-13
6. Ex. 22:25
7. Deut. 24:6
8. Deut. 24:17
9. Lev. 25:35-37
10. Deut. 23:20
11. Deut. 23:21