Well, with a help of generic math you can try implementing UDecimal based on Decimal if you like:

using System.Numerics;

...

public struct UDecimal 
    : IAdditionOperators<UDecimal, UDecimal, UDecimal>,    // We can add UDecimals
      ISubtractionOperators<UDecimal, UDecimal, UDecimal>  // We can subtract UDecimals etc.
{
    private readonly decimal _value;

    public UDecimal(decimal value)
    {
        ArgumentOutOfRangeException.ThrowIfNegative(value);

        _value = value;
    }

    public static UDecimal operator +(UDecimal left, UDecimal right) => 
        new UDecimal(left._value + right._value);

    public static UDecimal operator -(UDecimal left, UDecimal right) => 
        new UDecimal(left._value - right._value);

    ...
}

There are many interfaces you should implement, but almost all the methods are evident oneliners.

However, I suggest implementing your custom class, say

public struct Price {
  public decimal Value {get;}

  public Price(decimal value) {
    // Note some extra validation
    if (value < 0 || value > 1_000_000)
      throw new ArgumentOutOfRangeException(nameof(value));

    Value = value;
  }

  // Some extra property etc. 
  public decimal Vat => Value * 0.20M;
}

I would not try to implement a new numeric type but instead implement the logic on a higher level. Say, you have a bank account class. Then you can ensure that withdrawals don’t exceed the balance.

public class BankAccount
{
    public BankAccount()
    {
    }

    public BankAccount(decimal balance)
    {
        if (balance < 0) {
            throw new ArgumentException("Initial balance must not be negative");
        }
        Balance = balance;
    }

    public decimal Balance { get; private set; }

    public void Deposit(decimal amount)
    {
        if (amount <= 0) {
            throw new ArgumentException("Amount must be positive");
        }
        Balance += amount;
    }

    public void Withdraw(decimal amount)
    {
        if (amount <= 0) {
            throw new ArgumentException("Amount must be positive");
        }
        if (amount > Balance) {
            throw new InvalidOperationException("Balance too small for this withdrawal.");
        }
        Balance -= amount;
    }
}