What is the price that must be paid to the elder brother for building the house 25 years ago: Is it the value of the construction at that time, or its current value?

If the built house is owned by your brother and he left it for you to live in, either for free or for rent, and then he wished to sell it to you, the sale is completed at the price you both agree upon.

If the land is shared between you and your brother, none of the partners has the right to build on the shared land without permission. If one of the partners builds on the shared property without the permission of the others, and no partner requests division, and the other partner requests the removal of the building, then the building must be removed. If one of the partners requests division, the land is divided if it is divisible, because removing the building before division invalidates the builder's right. If the building falls within the builder's share after division, then that is acceptable. If it falls within another partner's share, then that partner has the right to demand its demolition, or to pay its value to the builder and own it. If part of the building falls within the builder's share and part within the other's share, the part belonging to the other partner is demolished at his request. If the demolition causes a decrease in the land's value, the builder guarantees it. If the demolition of the building is harmful to the land, the landowner guarantees the value of the building to the builder.