There are two types of profit:
Entrepreneurial profit, the reward for coming up with new ideas and taking risks implementing them, I don't think anyone thinks this type of profit is exploitation.
Capitalist profit: reward for owning capital which is what I think Marxists see as exploitation.
Ignoring inflation would you rather have £100 today or next year?
If you said today you can skip the next paragraph.
It is currently possible to get fixed rate bonds that have a yield greater than the rate of inflation. Therefore by consuming something today, you are sacrificing the ability to consume more in the future. Therefore people who given a free choice pick consumption today prefer consumption of less today than consumption of more in 5 of 6 years time. Furthermore, by owning a piece of land it is possible to hire some labour to produce an amount of saleable food with greater proceeds than the cost of labour. This way over an infinite period of time you could get infinite income from a given piece of land therefore if a time preference didn't exist land would sell at an infinite price (time preference is preferring something now than something later) which is obviously impossible so the land would only be sold against other lands. But since land has a finite price a time preference must exist. As to why such a preference exists that is a question of phycology.
So let's say I would rather have £100 now than £105 next year but you would rather have £105 next year than £100 this year we would benefit from you giving me £100 now in exchange for £105 next year.
Now let us imagine that a capitalist builds a steel mill costing £1 million that will last 11 years and produce £1.1 million a year of which £1 million a year is paid in wages. Are the workers exploited to the tune of £100,000? No, they are not because the capitalist pays the £1 million NOW and must wait for the profit and as we learned two paragraphs ago money now is more valuable than money later. Meanwhile, from the worker's point of view, they are saved from paying the £1 Million now and in exchange lose a yearly instalment of £100,000 for 11 years. From the point of view of the workers this is a preferable position, if it wasn't they would have built the factory or paid for its construction themselves. They prefer £1million now to £100,000 yearly for 11 years, this is because as established before Money now is more valuable than money later. The capitalist in this scenario has there preferences in the opposite order and prefers £100,000 per year for 11 years to £1 million now.
It is vain to say that this is wrong since the capitalists have more wealth than the workers so the workers couldn't afford to build the factory since even in the imaginary construction of a zero point where everyone has equal wealth and incomes different time preferences would almost certainly exist and through the process of time and voluntary payments of interest a capitalist system like we have today would evolve.
It is also vain to attempt to refute this idea using the labour theory of value since the ideas expounded here necessarily refute LTV and so to reference it in a refutation would be circular logic. (X is wrong because Y, Y is correct because X is wrong).