There is currently a rule that deals with negative numbers, which I will quote below.

This rule is somewhat vague on its definition of when negative numbers are and are not applicable. One specific example is that it states that if a calculation to determine

Rule 107.1b

Most of the time, the Magic game uses only positive numbers and zero. You can’t choose a

negative number, deal negative damage, gain negative life, and so on. However, it’s possible for

a game value, such as a creature’s power, to be less than zero. If a calculation or comparison

needs to use a negative value, it does so. If a calculation that would determine the result of an

effect yields a negative number, zero is used instead, unless that effect sets a player’s life total

to a specific value, doubles a player’s life total, sets a creature’s power or toughness to a

specific value, or otherwise modifies a creature’s power or toughness.

negative number, deal negative damage, gain negative life, and so on. However, it’s possible for

a game value, such as a creature’s power, to be less than zero. If a calculation or comparison

needs to use a negative value, it does so. If a calculation that would determine the result of an

effect yields a negative number, zero is used instead, unless that effect sets a player’s life total

to a specific value, doubles a player’s life total, sets a creature’s power or toughness to a

specific value, or otherwise modifies a creature’s power or toughness.

This rule is somewhat vague on its definition of when negative numbers are and are not applicable. One specific example is that it states that if a calculation to determine

**the result of an****effect**yields a negative number to use zero instead. However, if a calculation to determine a**payment**yields a negative number (say if you are at a negative life total and control Murderous Betrayal), the phrasing of this rule implies that the cost can be a negative amount. I think this rule needs clarification on where negative numbers are acceptable and where they are not.