What does 'contango' have anything to do with 'continue'?

Here, let us focus on this potential etymology, rather than the other one on Etymonline.

1853, "charge made or percentage received by a broker or seller for deferring settlement of a stock sale," a stockbroker's invention,
perhaps somehow derived from continue

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How does contango's definition below semantically appertain to the English verb 'continue'? What notions underlie 'continue', with contango's financial meaning below?

I quote Sundaram, Rangarajan, and Sanjiv Das. Derivatives: Principles and Practice. 2nd ed. New York, NY: McGraw-Hill Education, 2016, page 46.

A futures market is said to be in backwardation futures prices are below spot. It is said to be in contango if futures prices are above spot. As we will see in Chapters 3 and 4, in a typical commodity market with a positive cost-of-carry, the theoretical futures price is above spot, i.e., the market should be in contango. However, in some commodity markets (notably oil) futures prices are often below spot. This phenomenon is commonly attributed to the presence of a large "convenience yield" from holding the spot commodity, an issue we discuss further in Chapter 4.

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I quote Hull, J. C. (2022). Options, futures, and other derivatives (11th ed.), page 125.

When the futures price is below the expected future spot price, the situation is known as normal backwardation; and when the futures price is above the expected future spot price, the situation is known as contango. However, it should be noted that sometimes these terms are used to refer to whether the futures price is below or above the current spot price, rather than the expected future spot price.

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Op. cit., p 809.

Contango A situation where the futures price is above the expected future spot price (also often used to refer to the situation where the futures price is above the current spot price).

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Source for pic below.

The definitions you have listed below concerning futures markets are irrelevant from the point of view of the possible link between contango and continue. You need to look only at the first meaning of contango, a charge made for a deferred settlement.

One of the usages of continue is

1. to carry over, postpone, or adjourn;
   keep pending, as a legal proceeding.

Clearly the notion of postponing payment and demanding a charge for postponing payment are closely related.

From a linguistic point of view, the word 'contango' appears (to me) to be a sort of monster. Be it connected with 'continue' or with 'contingent' (Lat. con=cum + teneo or +tango), why is it not 'contingo'?

Independently on the actual meaning or usage of the word "contango" in English, according to etymoline:
„ ...perhaps somehow derived from continue or from Spanish contengo (I contain, etc ...)"

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The English „contango" cannot be directly derived neither from the English „continue" (< old French „continuer") nor from „contain" (< old French contenir).

Thus in my opinion either it really comes somehow from the Spanish contengo (< Latin contineo), or the origin or history of this word is simply unknown ...